1. Field of the Invention
The present invention relates to an electrophotographic toner for developing an electrostatic latent image formed in an electrophotographic image forming apparatus such as copiers, printers and facsimiles. Particularly, the present invention relates to an electrophotographic toner useful for an image forming apparatus using an intermediate transfer medium. In addition, the present invention relates to an image forming method using the toner.
2. Discussion of the Background
Electrophotographic image forming methods and apparatus using an intermediate transfer medium are well known. In the image forming methods and apparatus, the following image forming steps are typically performed:
(1) a visible image (i.e., a toner image) formed on an image bearing member is transferred on an endless intermediate transfer medium (first image transfer step);
(2) repeating the first image transfer step a plurality of times using different color toners, i.e., another toner image formed on the image bearing material is then transferred on the intermediate transfer medium on which the first image has been formed (another first image transfer step), if desired;
(3) the toner images transferred on the intermediate transfer medium are transferred on a receiving material to form a monochrome or color toner image on the receiving material (second image transfer step); and
(4) the toner image formed on the receiving material is fixed to form an image (fixing step).
In such image forming methods and apparatus, a problem which occurs is that the resultant toner image formed on a receiving material often has local image omission which is caused by image omission in the first and/or second transfer step. When such image omission occurs in a solid image, the image omission is observed as a white image having a certain area. When such image omission occurs in a solid line image, the solid line image is observed as a cut line.
In order to avoid such an image omission problem by improving the toner used, the following methods have been proposed:
(1) the fluidity of the toner is enhanced to improve the transferability of the toner in the first and/or second image transfer steps; and
(2) a particulate resin is added to the toner to avoid an aggregation problem of the toner when the toner is pressed in the first and second toner transfer step.
On the other hand, in attempting to avoid an offset problem in which a toner image on a receiving sheet adheres to a fixing roller when the toner image is fixed and the toner image is re-transferred on the receiving sheet and/or another receiving sheet, a method in which a silicone oil or a fluorine-containing oil is applied to the surface of the fixing roller constituted of a release material, such as silicone rubbers and fluorine-containing resins, to cover the surface of the fixing roller with the oil is typically performed. This method is effective for avoiding the offset problem, however, the method has a drawback such that the image forming apparatus becomes large in size because the apparatus has to have a device for supplying the oil to the fixing roller. In addition, this method has another drawback such that the life of the fixing roller is shortened because the oil causes peeling of the layers constituting the fixing roller.
In attempting to avoid such an offset problem without using such an oil supplying device, a technique in which a release agent such as low-molecular weight polyethylene and polypropylene is added to a toner to impart releasability to the toner when the toner is fixed is proposed.
However, the technique has a drawback such that the fluidity of the resultant toner deteriorates and thereby a film tends to form on the intermediate transfer medium, resulting in formation of image omission in the resultant toner image. In addition, other problems such that image density of the resultant toner image deteriorates, and toner adheres to a background area of an image, i.e., background development occurs.
In attempting to avoid such a filming problem (i.e., to improve the fluidity of a toner), the following techniques have been proposed:
(1) Japanese Laid-Open Patent Publication No. 3-243956 discloses a toner which has an average lattice length of primary peaks at low X-ray diffraction angles of from 200 to 5,000 xc3x85;
(2) Japanese Laid-Open Patent Publication No. 3-296067 discloses a toner in which a binder polymer and a polypropylene constitute an island-sea state, wherein the maximum value of the major axis of the island state which is formed by the polypropylene is from 200 to 3,000 xc3x85, and the average interval between an island and the adjacent island is not greater than 1 xcexcm;
(3) Japanese Laid-Open Patent Publication No. 5-45924 discloses a toner in which a release agent, which has a melting point of from 60 to 180xc2x0 C. and in which the difference between the melt starting temperature and the melt completing temperature is not greater than 50xc2x0 C., is formed on the toner in a thickness of from 100 to 5,000 xc3x85;
(4) Japanese Laid-Open Patent Publication No. 5-197199 discloses a toner in which a particulate polyolefin having a diameter of from 0.01 to 0.5 xcexcm is dispersed on the surface of the toner, wherein the concentration of the polyolefin in the toner is from 2 to 20% by weight (the toner is also intended to retain good-developing property and not to abrade a photoreceptor);
(5) Japanese Laid-Open Patent Publication No. 7-301951 discloses a toner which includes a binder resin and a release agent, wherein the difference in solubility parameter between the binder resin and the release agent is not greater than 1.5; and
(6) Japanese Laid-Open Patent Publication No. 7-271095 discloses a toner having a crystallinity of from 40 to 60% However, the techniques of (1), (2) and (3) mentioned above cannot perfectly avoid the adhesion of the release agent to the intermediate transfer medium.
In the technique (4), fine holes are formed on the surface of the toner using a particulate inorganic material to retain a release agent therein. Thus, the size of the dispersed release agent is controlled. To impart good releasability to the toner, the inorganic material has to be added in a relatively large amount, resulting in peeling of the inorganic material from the toner. The peeled inorganic material tends to adheres to an image bearing member, resulting in formation of black spots on the resultant toner image.
In the technique (5), the binder resin and the release agent mix compatibly and therefore the offset problem cannot be solved. In the technique (6), the filming of the release agent cannot be avoided when the dispersion of the release agent is insufficient.
Because of these reasons, a need exists for a toner which can produce good toner images without causing image defects such as image omission, decrease of image density and background development.
Accordingly, an object of the present invention is to provide a toner which can produce good toner images without causing image defects such as image omission, decrease of image density and background development.
Briefly the object and other objects of the present invention as hereinafter will become more readily apparent can be attained by a toner which includes toner particles including at least a binder resin having a first melting point and a first solubility parameter, a colorant, and a particulate release agent having a second melting point lower than the first melting point and a second solubility parameter different from the first solubility parameter, wherein the particulate release agent has an average needle-shape degree LD/SD of not less than 1.6, where LD is a diameter of a particle of the release agent dispersed in the toner in a major axis direction thereof and SD is a diameter of the particle in a minor axis direction.
Preferably, 75% by number or more of the particulate release agent included in the toner particles has a needle-shape degree not less than 2.0. In addition, 75% by number or more of the particulate release agent included in the toner particles preferably has an equivalent spherical particle diameter not greater than 1 xcexcm when it is assumed that the particulate release agent has a spherical shape.
Further, the melting point of the release agent is preferably from 65 to 100xc2x0 C., and the difference in solubility parameter between the binder resin and the release agent is preferably not less than 1.0. Such a release agent is preferably present in the toner in an amount of from 1 to 10% by weight.
Furthermore, the release agent preferably includes two or more release materials having a different solubility parameter and melting point.
In another aspect of the present invention, an image forming method is provided which includes the steps of forming a toner image on an image bearing member; first transferring the toner image on an intermediate transfer medium; optionally repeating the image forming and first transferring steps one or more times to form a color image on the intermediate transfer medium; and second transferring the toner image on the intermediate transfer medium to a receiving material, wherein the toner or toners are the toner of the present invention mentioned above. The second transferring step can be eliminated, i.e., the toner image on the image bearing member may be directly transferred onto the receiving material.
In yet another aspect of the present invention, a method for manufacturing a toner is provided which includes the steps of kneading a mixture of a binder resin having a first melting point and a first solubility parameter, a colorant, and a release agent having a second melting point lower than the first melting point and a second solubility parameter different from the first solubility parameter at a temperature not higher than a temperature higher than the first melting point by 20xc2x0 C. and higher than the second melting point using a kneader to prepare a mixture; subjecting the mixture to a cooling treatment; pulverizing the mixture; and optionally adding an external additive to the pulverized mixture to form a toner. The interval between the kneading step and the cooling step is not longer than 60 seconds.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.